Free-electron lasers are attractive as tunable sources of coherent radiation, and electromagnetic wigglers are singularly attractive to drive such lasers. However, the kinetic energy (or voltage) of electron beams which drive such wigglers at infrared and millimeter wavelength are often beyond the limits desirable for many applications. With high energy electron beams, necessary radiation shielding can become sizeable, and limits the use of wigglers in confined areas where space is at a premium, for example aboard military platforms such as ships or aircraft. Moreover, because the frequency of radiation depends on how fast the electron beam transverses the wiggler's spatial periods, one is naturally tempted to increase frequency by increasing electron beam energy. The use of harmonic laser operation, and smaller period wigglers, are being tested as ways to increase the frequency of radiated power without changing beam energy. Each has advantages and disadvantages, depending on the system or application. Microwigglers (wiggler period .lambda..sub.w &lt;5 mm) permit reduction of operating voltages proportional to (.lambda..sub.w).sup.1/2, with consequent reduction in shielding requirements, and cost. It has been difficult to fabricate small period wigglers with high magnetic field strength and uniformity with both a usefully large gap between opposite wiggler magnetic poles and good electron beam focusing.
Prior work shows that one can produce large wiggler flux intensities by immersing a periodic array of ferromagnetic material in a magnetic field, typically solenoid generated, which is directed axially along the wiggler, i.e., parallel to the direction of the electron beam's entry. The ferromagnetic material pulls the axially directed solenoid flux periodically up or down producing useful radially directed wiggler magnetic fields. This scheme combines the advantages of design simplicity with the ease of generating large solenoid fields, but also has the disadvantages of large residual axial fields within the wiggler, and large radial variations in the wiggler field away from the wiggler's axis of symmetry (the direction along which the electron beam enters the wiggler). Large axial magnetic fields can combine with radial fields to cause electron gyro-resonance, which can result in more intense radiation, but if the gyro-radius is too large, can cause the electron beam to strike and damage the wiggler structure.